Large dredgers are employed in hydraulic engineering and dredging operations. During the construction of such vessels, there is ongoing discussion between ship designers and the developers of the components required for dredging work. To make optimal use of the ship’s volume, some components are designed in a way that is less than ideal from a fluid mechanics perspective.
A concrete example of this can be found in the piping and pump systems on board. To minimise resistance in pipe flows, bends and connections must meet specific geometric requirements. For this reason, the flow following a bend should always be allowed to stabilise over a certain straight length of pipe, allowing the velocity across the pipe cross-section to homogenise again. In practice, however, these vessels often lack sufficient space, resulting in the use of excessively tight bend radii or the installation of pumps too close to the bend outlets.
FlowMotion has analysed the inflow to the various centrifugal pumps on board. This analysis was prompted by severe vibrations in the centrifugal pumps, which led to a drastically reduced service life. It was suspected that the upstream bends were creating an asymmetrical velocity profile at the pump inlet, leading to cavitation (the evaporation of water due to very low static pressures) on the pump blades.
Various pipe and bend configurations found on the fleet’s vessels were examined. Due to the difficulty of accessing these areas for conventional velocity measurement techniques, Computational Fluid Dynamics (CFD) simulations were used. The analyses demonstrated that bends have a highly detrimental effect on the velocity profile upstream of the pumps. The impact of this profile on the pump characteristic curve and cavitation behaviour was investigated further.
